Fuel cells directly convert chemical energy into electricity thereby eliminating the mechanical process steps that limit thermodynamic efficiency, and have been proposed as a power source for many applications. The fuel cell can be two to three times as efficient as the internal combustion engine with little, if any, emission of primary pollutants such as carbon monoxide, hydrocarbons and nitric oxides. Fuel cell-powered vehicles which reform hydrocarbons to power the fuel cell generate less carbon dioxide (green house gas) and have enhanced fuel efficiency.
Fuel cells, including PEM fuel cells [also called solid polymer electrolyte or (SPE) fuel cells], generate electrical power in a chemical reaction between a reducing agent (hydrogen) and an oxidizing agent (oxygen) which are fed to the fuel cells. A PEM fuel cell includes an anode and a cathode separated by a membrane which is usually an ion exchange resin membrane. The anode and cathode electrodes are typically constructed from finely divided carbon particles, catalytic particles supported on the carbon particles and proton conductive resin intermingled with the catalytic and carbon particles. In typical PEM fuel cell operation, hydrogen gas is electrolytically oxidized to hydrogen ions at the anode composed of platinum reaction catalysts deposited on a conductive carbon electrode. The protons pass through the ion exchange resin membrane, which can be a fluoropolymer of sulfonic acid called a proton exchange membrane. H2O is produced when protons then combine with oxygen that has been electrolytically reduced at the cathode. The electrons flow through an external circuit in this process to do work, creating an electrical potential across the electrodes. Examples of membrane electrode assemblies and fuel cells are described in U.S. Pat. No. 5,272,017.
Fuel cell processors (also known as fuel cell reformers) supply a hydrogen-containing gas stream to the fuel cell. Fuel cell processors include reactors that steam reform hydrocarbon feedstocks (e.g., natural gas, LPG) and hydrocarbon derivatives (e.g., alcohols) to produce a process stream enriched in hydrogen. Other by-products from the steam reforming of hydrocarbon include carbon monoxide and carbon dioxide. For example, methane is converted to hydrogen, carbon monoxide and carbon dioxide by the two reactions below:CH4+H2O→3H2+COCH4+2H2O →4H2+CO2 
The resulting gas is then reacted in the water-gas shift reactor where the process stream is further enriched in hydrogen by reaction of carbon monoxide with steam in the water-gas shift reaction:CO+H2OCO2+H2 
In fuel cell processors, the reaction is often conducted in two stages for purposes of heat management and to minimize the outlet CO concentration. The first of two stages is optimized for reaction at higher temperatures (about 350° C.) and is typically conducted using catalysts based on combinations of iron oxide with chromia. The second stage is conducted at lower temperatures (about 200° C.) and is typically conducted using catalysts based on mixtures of copper and zinc materials.
Other catalysts that can be used to conduct the water-gas shift reaction include platinum-based catalysts such as platinum on an alumina support or platinum on a cerium oxide support. While effective at producing hydrogen using the water-gas shift reaction when operated at temperatures above about 300° C., water-gas shift reaction catalysts also cause the formation of methane (CH4) by catalyzing the reaction of CO with hydrogen as shown below:CO+3H2→CH4+H2O.
This undesired side reaction sacrifices three moles of hydrogen for each mole of carbon monoxide converted to methane. Methanation can also occur under these conditions with carbon dioxide according to the equation shown below:
CO2+4H2→CH4+2H2O
In this side reaction four moles of hydrogen are consumed for each mole of carbon dioxide converted to methane. The production of methane during the water gas shift reaction (referred to herein as “methanation”) is a side reaction that consumes hydrogen gas in an exothermic reaction to ultimately reduce the hydrogen yield from the water gas shift reaction. Moreover, the methanation reactions accelerate with increasing catalyst bed temperatures. This property presents a liability, as the exothermic reaction can result in a runaway reaction with carbon dioxide, in addition to carbon monoxide, being methanated. Major hydrogen loss can occur and the catalyst can be damaged by high temperatures. In addition, methane is a greenhouse gas. The fuel cell is advertised as an emission-free energy producer, and release of methane is undesirable. Methane is difficult to combust during normal operating conditions of the fuel cell, so producing an appreciable quantity of methane is environmentally unfavorable.
Metals such as cobalt (Co), ruthenium (Ru), palladium (Pd), rhodium (Rh) and nickel (Ni) have also been used as WGS catalysts but are normally too active for the selective WGS reaction and cause methanation of CO to CH4 under typical reaction conditions. In other words, the hydrogen produced by the water gas shift reaction is consumed as it reacts with the CO present in the presence of such catalysts to yield methane. This methanation reaction activity has limited the utility of metals such as Co, Ru, Pd, Rh and Ni as water gas shift catalysts.
A need exists, therefore, for a method to produce a hydrogen-rich syngas, and catalysts which are highly active and highly selective for hydrogen generation at moderate temperatures (e.g. below about 450° C.) to provide a hydrogen-rich syngas from a gas mixture containing hydrogen and carbon monoxide.